TISSUES OF ANQIOSPERMS 



443 



Fig. 320.— Cross-section of 

 tliestem of wliicli Fig. 319 

 is tile diagram, taicen above 

 tlie flf til leaf .—After Nageli. 



tweea the fifth aud sixth leaves of the preceding figure. The 

 bundles are numbered as in Pig. 319. 



542. — In a comparatively small number of instances there 

 are fibro-vascular bundles in the stem which have no connec- 

 tion with the leaves. These are known as cauline bundles. 



543. — In the Monocotyledons and 

 many herbaceous Dicotyledons, the 

 fibro-vascular bundles are closed — that 

 is, there is no zone of meristem tissue 

 left between the xylem and phloem after 

 these have passed over into permanent 

 tissues. There is, as a consequence, a 

 definite period of growth for the bun- 

 dles, and when any bundle has fully 

 formed all its tissues, no further devel- 

 opment can take place in it. This gen- 

 erally results in definitely limiting the growth of the inter- 

 nodes, and in consequence such plants are as a rule short- 

 lived. The perennial woody-stemmed Dicotyledons, and 

 some of the herbaceous annuals, possess bundles which are 

 open — that is, there is left between the xylem and the phloem 



a zone of meristem tissue which 

 continues to grow long after the 

 other parts of the bundle have 

 passed over into permanent tis- 

 sues. Plants with such bundles 

 may live and continue to grow for 

 an indefinite time. 



544. — A cross-section of the 

 stem of a Palm (Pig. 321) shows 

 it to be composed of parenchyma- 

 tous tissue traversed by myriads 

 of fibro-vascular bundles, which 

 descend from the crown of leaves. 

 Each leaf sends down from its broad insertion numerous 

 bundles, which, in a vertical section, are seen first to pass in 

 toward the centre of the stem, and then to curve downward 

 and finally outward. The centre of the stem is thus softer 

 than the peripheral portion, as in the latter the descending 



Fig. 321. — Cross-section of the 

 stem of a palm, ec, cortical zone ; 

 Ig, tile softer interior portion of the 

 stem ; lg\ the harder peripheral 

 portion. — After Diichartre. 



